scale

Most of the units we use to measure everyday things here on Earth are not much help when we are dealing with the size and scale of stars and space. That's because objects in space are usually so big or so far away that the numbers involved are huge.

How far away are the Earth and the planet Pluto from the Sun?
Since we are used to using kilometres as a measure of distance, we can say that the Earth is about 150,000,000 (one hundred and fifty million) kilometres from the Sun and Pluto is about 6,000,000,000 (six billion) kilometres away.

However, to make things easier, why don’t we invent a new unit so that the Earth is just one of those units from the Sun? That's exactly what astronomers have done and they call it the Astronomical Unit or AU for short. In those units the Earth is 1 AU from the Sun and Pluto is just 40 AU from the Sun. That sounds easier to use and is great for talking about distances in our solar system.

The nearest star to the Sun is called Proxima Centauri. It’s just over 271,000 AU away and is a very faint star. Betelgeuse, the brightest star in the constellation of Orion, is a whopping 32,800,000 AU away. Things are starting to get awkward again - we need a new unit.

Unfortunately, when we move on to talk about the distance of the stars we run into trouble again.

We know that light travels very quickly in space (300,000 km per second). Why not have a new unit which is the distance light travels in one year and call it a Light Year (LY for short)? How far is a light year? It's 9,460,000,000,000 km or 63,067 AU.

You will find the light year used a lot in astronomy. Don't forget that it is a DISTANCE not a time and it's a very long way.